The three-hour musical drew 18.6 million total viewers Thursday night as Underwood sang the part of Maria and "True Blood's" Stephen Moyer performed as Captain Von Trapp, the Los Angeles Times reported Saturday.
The large audience draw could foreshadow a shift in programming for networks, which are competing for viewers against Netflix, Amazon and even mobile phone apps like Instagram, the newspaper said.
"There is an edge to doing something live on TV [rather] than something pre-recorded, and it showed," said Brad Adgate, an analyst at ad firm Horizon Media in New York. "Anyone familiar with the movie saw different scenes from the Broadway show."
This type of live programming could be especially attractive for advertisers, which are seeing viewers fast forward through commercials as they watch shows on DVRs, the Times said.
"I thought it would have done 10 to 12 million ... not the 18-plus million," Adgate said. "We will definitely see more of this based on the ratings."